The goal of this work has been to develop a sensitive quantitative in vitro focal immunoassay for human immunodeficiency virus (HIV) and HIV-infected cells. This has been approached by inserting the human CD4 gene into human cell lines. Clones with high CD4 expression were then used as targets for HIV infection which was detected by HIV-specific antibodies with indirect immunofluorescence or immunoperoxidase methods. The assay was shown to be HIV-specific by using monoclonal antibodies specific for HIV envelope or core proteins; however, polyclonal human anti- HIV serum was also effective. The assay was highly sensitive, capable of detecting most wild-type HIV strains isolated from patients. Future experiments are aimed at using the assay to study HIV expression in cells of AIDS patients.